The Infrared Spectral Properties of Magellanic Carbon Stars

Abstract

The Infrared Spectrograph on the Spitzer Space Telescope observed 184 carbon stars in the Magellanic Clouds. This sample reveals that the dust-production rate (DPR) from carbon stars generally increases with the pulsation period of the star. The composition of the dust grains follows two condensation sequences, with more SiC condensing before amorphous carbon in metal-rich stars, and the order reversed in metal-poor stars. MgS dust condenses in optically thicker dust shells, and its condensation is delayed in more metal-poor stars. Metal-poor carbon stars also tend to have stronger absorption from C 2 H 2 at 7.5 µm. The relation between DPR and pulsation period shows significant apparent scatter, which results from the initial mass of the star, with more massive stars occupying a sequence parallel to lower-mass stars, but shifted to longer periods. Accounting for differences in the mass distribution between the carbon stars observed in the Small and Large Magellanic Clouds reveals a hint of a subtle decrease in the DPR at lower metallicities, but it is not statistically significant. The most deeply embedded carbon stars have lower variability amplitudes and show SiC in absorption. In some cases they have bluer colors at shorter wavelengths, suggesting that the central star is becoming visible. These deeply embedded stars may be evolving off of the AGB and/or they may have non-spherical dust geometries.

An Infrared Census of DUST in Nearby Galaxies with Spitzer (DUSTiNGS). IV. Discovery of High-redshift AGB Analogs*

“…For AGB stars, the chief uncertainty lies in the metallicity dependence of dust production. Some studies show a clear metallicity effect in carbon-rich AGB stars (van Loon et al 2008), while others suggest that C star dust production has weak-to-no metallicity dependence (Sloan et al 2012(Sloan et al , 2016Boyer et al 2015c). It is, however, the dusty M-type AGB stars that may be more important at early times because the most massive AGB stars (and thus those that can produce dust on the shortest timescales) are O-rich.…”

Characterizing the Population of Bright Infrared Sources in the Small Magellanic Cloud

“…The IRS peaked up on this object rather than the intended target. Fortuitously, both objects were in the SL slit, so those data for MSX SMC 125 could be included in the carbon star overview study of Sloan et al (2016).…”

“…This program included 35 targets from the MSX SMC catalog and one additional target known to be oxygenrich from its optical spectrum 2 (Smith et al 1995). These targets include 19 carbon-rich stars on the asymptotic giant branch (AGB; Sloan et al 2006bSloan et al , 2016, two R CrB candidates (Kraemer et al 2005), one carbon-rich post-AGB object (Kraemer et al 2006), and two young stellar objects (YSOs; Oliveira et al 2013). This paper examines the remaining 11 sources in the sample 3 .…”

“…We combined IRAC 4.5 µm data with WISE 4.6 µm data (W2) similarly. Sloan et al (2016) calibrated color corrections between these filter pairs for carbon stars, but the present MSX sample contains many other source types. The greater overlap between W2 and the 4.5 µm IRAC filter leads to smaller corrections compared to W1 and 3.6 µm.…”

“…The infrared spectral classifications follow the Hanscom method, as described by Kraemer et al (2002b) and revised by Sloan et al (2008Sloan et al ( , 2016 for spectra from the IRS. In this system, the numbered group indicates the overall color of the spectral energy distribution in the midinfrared, from 1 for blue sources with stellar continua to 5 to very red sources with cool dust emission.…”